1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program for intercommunication between a plurality of wireless stations. In particular, the present invention concerns a wireless communication apparatus, a wireless communication method, and a computer program for expanding transmission capacity by carrying out MIMO (Multi Input Multi Output) communication which utilizes spatial multiplexing and forms a plurality of logical channels, pairing a transmitter having a plurality of antennas with a receiver having a plurality of antennas.
More specifically, the present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program for reducing power consumption during reception of spatially multiplexed MIMO signals. In particular, the present invention concerns a wireless communication apparatus, a wireless communication method, and a computer program for reducing power consumption during packet detection standby and power consumption during reception of spatially multiplexed signals.
2. Description of the Related Art
Computer networking including a LAN can efficiently realize the sharing of information resources and the sharing of equipment resources. Nowadays, attention is being given to a wireless LAN as a system that frees users from cabling of conventional wired LANs. The wireless LAN can eliminate most of cables from workspaces such as offices. Accordingly, it is possible to relatively easily move communication terminals such as personal computers (PCs).
In recent years, there is remarkably increasing demand for wireless LAN systems as they achieve higher speeds and become available at reduced costs. Particularly, introduction of a personal area network (PAN) is being considered to construct small-scale networks for information communication between electronic devices available around users. For example, some different wireless communication systems and wireless communication apparatuses have been standardized by utilizing such frequency bands as 2.4 GHz and 5 GHz which require no license from an administrative government office.
Canonical standards concerning wireless networks can include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see non-patent document 1), HiperLAN/2 (e.g., see non-patent document 2 or 3), IEEE802.15.3, and Bluetooth communication, for example. The IEEE802.11 has enhanced standards such as IEEE802.11a (e.g., see non-patent document 4), b, g . . . depending on differences of wireless communication systems and frequency bands.
IEEE802.11a supports a modulation scheme for achieving a maximum communication speed of 54 Mbps. However, there is desired a standard for realizing a higher bit rate for the communication speed. For example, in IEEE802,11n, with the aim of establishing a wireless LAN technology for realizing a high speed exceeding an actual throughput of 100 Mbps, the next generation of wireless LAN standards is being developed.
As a technology for realizing a higher speed of wireless communication, MIMO (Multi-Input Multi-Output) communication is coming to attention. This is a technology for expanding transmission capacity and achieving improvement in communication speed by realizing spatially multiplexed transmission channels (hereinafter also referred to as “MIMO channel”) with a plurality of antenna elements at a transmitter and a receiver respectively. In the MIMO communication, good frequency utilization efficiency is obtained due to utilization of spatial multiplexing.
FIG. 16 schematically shows a MIMO communication system. As shown in FIG. 16, a transmitter and a receiver are equipped with a plurality of antennas respectively. The transmitter performs space-time coding on a plurality of transmission signals, which are then multiplexed, distributed to M antennas, and transmitted to a plurality of MIMO channels. The receiver performs space-time decoding on reception signals received by N antennas via the channels to obtain reception data. In this case, a channel model is composed of a radio wave environment around the transmitter (transfer function), a structure of channel space (transfer function), and a radio wave environment around the receiver (transfer function). In the case of multiplexing signals transmitted from each antenna, there occurs crosstalk. However, by processing signals at the receiver, each multiplexed signal can be extracted properly without crosstalk.
In a MIMO communication scheme, the transmitter distributes transmission data to a plurality of antennas and transmits it through a plurality of spatially-multiplexed virtual MIMO channels, and the receiver obtains reception data by processing signals received by a plurality of antennas. In this manner, the MIMO communication scheme utilizes channel characteristics and differs from a mere transmission/reception adaptive array.
There are proposed a variety of configuration schemes for realizing MIMO transmission. However, it is a big issue on implementation how channel information is exchanged between a transmitter and a receiver in accordance with an antenna configuration. In the case of exchanging the channel information, it is easy to perform a method of transmitting known information (preamble information) only from the transmitter to the receiver. In this case, the transmitter and the receiver are independent of each other and perform spatial multiplexing transmission. This is called an open-loop type of MIMO transmission scheme. As an extension of the open-loop type, there is a closed-loop type of MIMO transmission scheme for producing ideal spatial orthogonal channels between the transmitter and the receiver by feedback of preamble information also from the receiver to the transmitter.
The open-loop type of MIMO transmission scheme can include V-BLAST (Vertical Bell Laboratories Layered Space Time) scheme for example (e.g., see patent document 1). Further, as an ideal form for the closed-loop type of MIMO transmission, there is known an SVD-MIMO scheme utilizing singular value decomposition (SVD) of a propagation path function (e.g., see non-patent document 5).
In this manner, according to the MIMO transmission scheme, by using a plurality of spatially multiplexed channels in parallel between the transmitter having a plurality of transmit antennas and the receiver having a plurality of receive antennas, transmission speed between the transmitter and the receiver can be improved using a limited bandwidth.
However, from the viewpoint of power consumption at the receiver, the introduction of the MIMO transmission scheme brings about a problem. In the case of receiving a signal from one transmission device (no introduction of a MIMO technology), one reception device is generally enough to work. Assume that power consumption at the receiver in this case is represented by P [W]. On the other hand, in the case of a MIMO receiver having n reception branches, the electric power for reception comes to P×n[W], with a simple calculation. That is, in the MIMO transmission scheme, an increase in the number of branches forms more MIMO channels which can realize higher-speed transmission with a limited bandwidth; however, the power consumption increases in rough proportion to the number of branches.
This is not particularly a problem for devices powered from external power supplies such as stationary TV and PC. However, this affects usability of portable devices powered from batteries such as PDA (Personal Digital Assistant) since the battery life varies with the power consumption. Further, from the viewpoint of the environment or the social ecology, a power saving is also required for stationary devices powered from commercial AC power.
It is unavoidable that the power consumption increases in rough proportion to the number of branches at the time of receiving spatially-multiplexed signals with MIMO in reality. However, since it is necessary that a wireless communication apparatus is in a reception standby state or watches statuses of media even during no data transmission, there is a problem on the power consumption. That is, a receiver spends most of the time on packet detection, and the wasted power becomes excessive if the receiver goes on reception standby with a plurality of reception branches during this period.
[Patent document 1] Japanese Unexamined Patent Publication No. Hei 10-84324
[Non-patent document 1] International Standard ISO/IEC 8802-11:1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent document 2] ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part1: Basic Data Transport Functions
[Non-patent document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (RLC) sublayer
[Non-patent document 4] Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band
[Non-patent document 5] http://radio3.ee.uec.ac.jp/MIMO (IEICE_TS).pdf (as of Oct. 24, 2003)